The present invention relates to a method and a device for monitoring the energy and/or the position of a pulsed and scanned laser beam. Specifically, the present invention relates to a method and a device for measuring the energy of a pulsed laser beam to determine the actual value for controlling the energy and, at the same time, to test the full functionality of a beam deflection unit and the alignment of the optics of an ophthalmologic excimer laser for corneal surgery.
Laser energy is measured using optical photodiodes, pyroelectric or thermophilic sensors. To this end, generally, part of the laser radiation is guided via a splitting mirror (splitter) or a glass plate to a sensor. The energy measured there is then considered to be proportional to the energy that is applied in the treatment field, according to the division ratio during the coupling out of the radiation. Here, energy losses of changing transmission conditions of the following optical system because of defective optics or increasing absorption in air are not detected or compensated for.
When working with a beam deflection unit (scanner), the proper functioning of the scanner can be accomplished by measuring the position of the scanner mirrors. In this context, the position of the mirror is determined by capacitively or optically measuring the position of the mirror support. If, in the case of this independent capacitive or optical position measurement in the scanner (closed loop), the nominal position changes, the scanner feeds back when the corresponding position is reached (position acknowledge signal). In the case of commercially available scanners, however, the position resolution of the independent position monitoring is too low so that the position acknowledge signal is not changed in the case of very small deflections or changes in the nominal position. Thus, moreover, the actual position of the laser beam cannot be determined either. If the optical beam guidance system is out of alignment, for example, due to a defective scanner mirror, this is not detected. If an error occurs during setpoint selection, for example, because of a corrupted setpoint signal due to electric disturbances or because of a missing signal due to a cable break, this cannot be detected either because the scanner automatically positions itself according to the false signal and feeds back the reached position or because the scanner maintains its old position and continues to give the false impression of a correct position.
Moreover, it is conceivable to determine the position of the laser beam by measuring an auxiliary beam in the visible range via an image sensing unit. These errors of the position monitoring of the beam deflection unit can be detected using an image-processing system in that the position of a visible aiming beam in the treatment field is measured by a camera and evaluated. The actual position can then be compared to the setpoint input. However, the potential processing speed of the laser system is limited by the image repetition rate of the camera and the relatively high dead time during image analysis. The dead time essentially depends on the processing speed or computing power of the image-processing system and further reduces the processing speed of the overall system. It is not possible to determine the energy at the same time.